Let's say you have an AJAX call, and some other function that depends on the AJAX call loading before it runs.

Here's the AJAX function.

function doTheThing() {
  $.ajax({
    url: window.location.href,
    type: 'POST',
    data: {
      key: 'value',
    },
    success: function (data) {
      console.log(data)
    },
    error: function (error) {
      console.log(error)
    },
  })
}

And you might call the two functions in succession, and find that the second function doesn't work, because it depends on the first one.

doTheThing()
doSomethingElse()

We can quickly and easily rewrite this with a Promise

function doTheThing() {
  return new Promise((resolve, reject) => {
    $.ajax({
      url: window.location.href,
      type: 'POST',
      data: {
        key: 'value',
      },
      success: function (data) {
        resolve(data)
      },
      error: function (error) {
        reject(error)
      },
    })
  })
}

Now we can do the AJAX call, run the success function, and follow it up with any subsequent code.

doTheThing()
  .then((data) => {
    console.log(data)
    doSomethingElse()
  })
  .catch((error) => {
    console.log(error)
  })

And that's it!